I don't know where to buy on the internet but, around here (NC) floor tread can be purchased from about any metal service center.
Ryerson Tull has service centers in Stockton and LA.
Check your yellow pages or the Thomas Register under "Aluminum" for service centers in your area.
You may even find it at a local sheet metal fabrication shop but you will pay more for it (if they sell to the public). They could also shear it to size and form it.
(I am a Manufacturing Engineer at a precision metal fabricator at my full time job).
I hope this helps.
James
1st Class Lawn Service